Ultrastructural studies of pial vascular endothelium following damage resulting in loss of endothelium-dependent relaxation.

The changes in pial arterioles of 7 cats were examined by electron microscopy after injury that eliminates endothelium-dependent relaxation to acetylcholine or bradykinin. The injury was produced by exposing the vessels to mercury light in situ in the presence of intravascular sodium fluorescein dye. Previous studies showed that, at the time of initial injury and loss of endothelium-dependent responses, the endothelial cells displayed minimal ultrastructural evidence of injury. Because these changes might indicate the beginning of a sequence of irreversible alterations representing or leading to cell death, the present study was carried out 31/2-4 hours later, when ultrastructural evidence of progressive cell degeneration should readily be recognized. No such changes were observed. Instead, most vessels showed only the minimal alterations observed initially (endothelial vacuolation, blebs, and lucencies). Four of 19 vessels were completely normal. The findings fail to support the hypothesis that irreversible cell damage or death caused by the light + dye injury has caused the associated loss of endothelium-dependent relaxation. Rather, the findings support the concept that much lesser degrees of trauma are sufficient to impair the dilating responses of cerebral microvessels. This greatly expands the potential spectrum of pathologic states that might result in loss of endothelium-dependent relaxation.